\documentclass[aspectratio=141,9pt,xcolor={svgnames,x11names},fleqn,usepdftitle=false,bookmarks=true]{beamer}

\begin{frame}[plain]
  \begin{center}
    \bgroup
      aa
    \egroup
  \end{center}
\end{frame}

\begin{frame}[plain]
\begin{center}
\begingroup
  aa
\endgroup
\end{center}
\end{frame}

\end{document}
